INSECT DEFOLIATION AND NITROGEN CYCLING IN FORESTS

Lovett, et al., 2002

Presented by: Chelsea Krieg

OUTLINE

• Introduction: Defoliation

• Shift in Foliar N Budget due to Defoliation

• Invertebrate N Processing Efficiency

• Frass: N Immobilization Agent

• Fate of Immobilized N

• N Export

• Factors that May Affect N Loss

• Summary

INTRODUCTION

Defoliation can cause the:

Direct Impact

• Decrease in transpiration and tree growth

• Changes allocation of C within the tree

• Decrease in seed production

• May lead to increase chemical defenses against insects

Indirect Impact

• Increase in tree mortality, light penetration to the forest floor, and water drainage

• Shifts in tree species composition within the population

• Changes in population size of insect predators

• N export through drainage network (leaching of soil-available N) and acidification down network

• Not really the case here- find tight internal cycling shift, but little loss

Defoliation differs from other disturbances generating N loss through drainage Network:

1. Trees can usually survive initial damage, so their woody tissue and structure remains intact

2. Soil minimally disturbed, so rates of erosion are not heavily altered

3. If trees are not killed, recovery is possible within time span of weeks, instead of years

Argument: Defoliation disturbance leads to N redistribution rather than N loss

INTRODUCTION

Normal N-Cycle in Hardwoods:

1. N withdrawn from tree N-stores are utilized to make

leaves

2. Leaves take part in photosynthesis, normal root

uptake

3. Change of season, tree resorbs bulk of N before

abscission of leaves

4. Any remaining N required by the tree is retrieved by

roots

Happy healthy trees

In Defoliation, N retrieval from leaves is

interrupted and tree must rely on root N uptake

SHIFT IN FOLIAR N BUDGET

In Defoliation, foliar N is diverted to insect biomass, frass

(insect excrement), and green leaf fall, instead of remaining in

tight internal tree N cycle. The tree is not able to resorb N

and must rely on root N uptake.

Gypsy Moth

LEPIDOPTERAN N EFFICIENCY

• Even with alkaline midgut Gypsy Moths (Lymentria dispar L)

produces acidified frass

• Instar nymphs likely experience ammonia volatilization in hindgut, but

effectively prevent N gas loss through ion pumping

• Loss 84% N consumed through frass (insect feces)

• Also, sloppy eaters

FRASS: IMMOBILIZATION AGENT

Labile C in Frass Increase in Microbial Growth leading to Immobilization of N,

Decrease in Mineralization (less Inorganic N available for vegetation)

Other examples:

• Elm spanworm frass immobilized inorganic N when incubated in stream water

• In NC experiment, frass additions had no effect on NO3- and PO43- availability

• Aphid honeydew suppressed N mineralization rates through similar mechanism

Gypsy moths defoliate in early summer (June-July)

• Trees need following months to adjust for defoliation damage,

so low N availability (potentially caused by immobilization) can hinder tree recovery

Litter Plot

• Most 15N stayed in undecomposed

litter, small amount of 15N mobilized

into soil

• Retained less 15N than frass plot, but

what was retained remained in surface

soils potentially being more available

to plants

FATE OF IMMOBILIZED N

<0.01% of applied 15N was leached from the systems.

Defoliation disrupts the normal soil-plant N system, but each N cycle is tightly conserved by employing very different mechanisms.

Frass Plot

• Frass dissolved quickly and moved

to all of the measured soil profile

• 99% of 15N recovered in soil

• 1% of soil N was readily available

(inorg N, microbial, or

mineralized)

N is retained in the litter and is released slowly during litter

decomposition

N moves to the subsoil and its

readily retained

N EXPORT

Other studies have shown N exports after defoliation

N export not from foliar-frass cycling, but due to:

• atmospheric deposit rates

• N mobilized by defoliation

Coweeta and the Cankerworm

• Increase in stream water nitrate, but still very low (<0.5kg/ha-1/yr-1)

• <2% insect-generated N deposition lost via leaching

Hubbard Brook and Heterocampa

• Defoliation has “no effect” on

stream N export

• ~20% of watershed was

defoliated

Virginia and Lymentria

• >90% of leaf area removed

• Previously undetectable,

Nitrate exports 4kg/ha-1/yr-1!

• Annual N deposition for this

region is 8kg/ha-1/yr-1 , so

watershed experienced net

retention of N.

FACTORS THAT MAY GENERATE N LOSS

Hydrologic Bypass

• Precipitation pulse after

defoliation frass deposit can

transport frass away from

watershed and prevent

reinvestment of frass N

• Sandy soils or soils with more

channelized structure can

experience less contact with

frass N

Tree Mortality

Death of Trees:

• Reduce uptake of N mobilized

from frass

• Reduce sink large sink for pool

of N recycled from decomposing

OM annually

• Contribute another large source

of N from decaying roots

Low N retention in Soils

• Factors producing low SOM

(Thin soils, etc) will affect the

retention of frass N

• N retention also affected by N

saturation and N need

SUMMARY

1. Defoliation leads to N redistribution rather than N loss

2. In the case of Gypsy Moths, the ecosystem loses very little N by volatilization from the insects themselves, but frass becomes an important pathway for internal N-cycling.

3. N delivered to the forest floor by frass pellet trigger rapid immobilization through microbial growth and experience long term retention in soil organic matter (SOM)

4. Trees surviving defoliation are capable of N uptake through their roots, but may have to struggle with low N availability

5. Defoliation disrupts the normal soil-plant N system, but the pre and post-defoliation N cycles are tightly conserved by employing very different mechanisms.

6. N export after defoliation can still occur after defoliation

QUESTIONS

• If it weren’t as interdisciplinary, would we have had the same level of insight?

Were there any disciplines that could have been included?

• What limitations did the plot experiment have?

• If there were a large storm that knocked down trees during a defoliation

period, how do you think that would affect immobilization and mineralization?